If you've taken your dog or cat to the veterinarian recently — especially during the holidays — chances are it was for stomach upset. That's one of the most common problems veterinarians see in pets, and not just during holidays, but year-round.

Vomiting and diarrhea are obvious signs of intestinal upset, but you may also notice appetite loss, weight loss, blood in the vomitus or stool, or even more subtle clues, such as changes in attitude or decreased energy levels. But because these signs can indicate any number of disease states, getting to the root of the problem can require high-level detective skills on the part of your veterinarian. Possible causes include viral or bacterial infections, dietary indiscretions (aka garbage gut), ingestion of toxic substances, intestinal obstructions, allergic reactions, parasites and Addison's disease.

"One of the most common things we see in both dogs and cats is something termed chronic enteropathy, which refers to conditions of the intestinal tract that result in gastrointestinal signs of at least three weeks duration," said internal medicine specialist Sara Wennogle, a veterinarian at Colorado State University Veterinary Teaching Hospital. "However, we only arrive at the diagnosis of chronic enteropathy after the exclusion of a lot of other common causes of these clinical signs."

One of the diseases that must be excluded before a diagnosis of chronic enteropathy is Addison's disease. Certain indicators from the pet's history, breed or lab work will suggest the need to screen for other diseases as well.

Veterinarians typically begin by excluding the most obvious suspects. They'll ask whether your pet's diet has changed recently or if he has gotten into the trash or been somewhere that he could access something toxic, and they may order a fecal exam to screen for intestinal parasites such as hookworms, roundworms, Giardia and Cryptosporidium.

Once the basic baddies are ruled out, your veterinarian may pull out the big guns: complete blood count, serum biochemistry panel, urinalysis to check for evidence of kidney disease or imaging such as abdominal ultrasound or X-rays to see if there's a foreign object causing a blockage or an intestinal mass.

Certain indicators from lab work or the animal's breed may suggest screening for Addison's disease. For instance, Wennogle says, a 5-year-old poodle should probably have Addison's excluded. No single test can indicate that a pet has, say, inflammatory bowel disease.

The diagnostic testing of chronic gastrointestinal signs in pets can be lengthy and costly. Fortunately, not every pet with chronic gastrointestinal signs requires a $2,000 workup. In many cases, a simple change in diet can solve the problem. A large proportion of both dogs and cats have a positive response to dietary change. It's a mainstay of therapy in managing chronic GI disease.

"There's good evidence that dietary constituents will contribute to gut inflammation," Wennogle says. "Therefore, manipulation of the diet has a lot of value in treating underlying diseases." Some diets available from veterinarians contain prebiotics or probiotics that can be helpful or alterations in the fatty acid ratio that can help modulate inflammation.

Another tip: Take your pet to the veterinarian sooner rather than later for signs of gastrointestinal disease.

"We have a better chance for cure the earlier the client can bring in the dog or cat," said M. Katherine Tolbert, a veterinary internal medicine specialist at Texas A&M University College of Veterinary Medicine & Biomedical Sciences. "It takes a long time to get control of chronic disease. If the disease has been going on for months, we often can't achieve remission within one week."

Take pets to the veterinarian immediately for sudden signs of severe illness such as weakness, continual retching and inability to keep down water, says Michael Stone, a veterinary internal medicine specialist at Tufts University's Foster Hospital for Small Animals in North Grafton, Massachusetts. Don't let vomiting go on for more than six hours, and take pets in right away if they appear weak.

Pet Connection is produced by a team of pet-care experts headed by veterinarian Dr. Marty Becker and journalist Kim Campbell Thornton of Vetstreet.com. Joining them is dog trainer and behavior consultant Mikkel Becker.